The present invention relates to cancelers for use in sidelobe canceler systems, MTI systems and other systems that decorrelate signals by removing correlated components, and more particularly to digital cancelers.
Generally a signal-processing system is designed to reduce the presence of undesired signals received by its receiver. An example of apparatus to reduce undesired signals is a canceller. A canceller may be used for cancelling radar clutter that is correlated from pulse to pulse in an MTI system, for cancelling interference entering a radar's sidelobes in a sidelobe canceller system, and for similiar applications.
Commonly-assigned U.S. Pat. No. 4,086,592 discloses a complex data, digital-complex-weight canceler for a radar system whose receiver includes a local reference oscillator to convert correlated main and auxiliary channel inputs down to baseband frequency range, by mixing the inputs with a reference signal at the carrier frequency of the main input signal, and in addition derive the inphase I and quadrature Q vector components of each input. Generally, the I and Q components of each input are sampled at a rate equal to the main channel signal bandwidth (the highest difference frequency) and converted to the real and imaginary parts of complex digital numbers so that the input to the canceler from the main channel is a dual input comprising a complex digital number, and the input to the canceler from the auxiliary channel is a dual input comprising another complex digital number.
The canceler includes a conjugator, first and second complex multipliers, first and second complex summers, a complex divider, and a third complex multiplier to derive an optimum complex weight for the inputs which is subtracted from the complex main channel input to obtain an uncorrelated main channel output signal.
In microminiature circuits, the number of multipliers, summers, dividers, etc. used is a significant factor in determining the size and economy of the device. The use of a complex weight and complex arithmetic in the canceler disclosed in U.S. Pat. No. 4,086,592 substantially increases its size and cost. For example, a single complex multiplier requires four multipliers, two adders, and one subtractor, in its construction. A design which could significantly reduce the number of multipliers, summers, dividers etc. would enjoy an economic advantage due to that factor alone.